Wireless communication systems are constantly under development. Developing systems provide a cost-effective support of high data rates and efficient resource utilization. One communication system under development is the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 8. An improved version of the Long Term Evolution radio access system is called LTE-Advanced (LTE-A). The LTE is designed to support various services, such as high-speed data, multimedia unicast and multimedia broadcast services.
In wireless communication systems, multiple antennas can be used in reception and/or transmission for improving link reliability and/or increasing transmission rate. In solutions based on multiple antennas, precoding can be used to improve performance of a multiple antenna system. Precoding is usually used in transmitters to mitigate distortion introduced by channel response and/or equalizers used in receivers.
There are two common ways of realizing precoding. Precoding may be realized as a codebook-based or non-codebook-based solution. A transmitter and a receiver may have a common precoding codebook, i.e. a finite collection of precoding vectors or precoding matrices. The receiver, for example a mobile station, typically decides which vector or vectors of the codebook are selected for use and feedbacks its index to a transmitter, for example an eNodeB, on a feedback channel.
In non-codebook precoding, channel state information (CSI) is usually furnished to a transmitter by using a feedback signal from a receiver in a frequency division duplex (FDD) mode or in time division duplex (TDD) mode by using the reciprocity principle. Alternatively, in FDD mode, the receiver may decide on antenna weighting and feedback this information via a feedback channel.